Crystal pulling systems having a cover member for covering the silicon charge and methods for growing a melt of silicon in a crucible assembly

ABSTRACT

Crystal pulling system having a housing and a crucible assembly are disclosed. The system includes a heat shield that defines a central passage through which an ingot passes during ingot growth. A cover member is moveable within the heat shield along a pull axis. The cover member may include an insulation layer. The cover member covers at least a portion of the charge during meltdown.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 63/073,180, filed Sep. 1, 2020, which is incorporatedherein by reference in its entirety.

FIELD OF THE DISCLOSURE

The field of the disclosure relates to crystal pulling systems forgrowing a monocrystalline ingot from a silicon melt and, in particular,crystal pulling systems that include a cover member for use incontinuous Czochralski silicon ingot growth.

BACKGROUND

Silicon crystal silicon ingots may be prepared by the Czochralski methodin which a single crystal silicon seed is contacted with a silicon meltheld within a crucible. The single crystal silicon seed is withdrawnfrom the melt to pull a single crystal silicon ingot from the melt. Theingot may be prepared in a batch system in which a charge ofpolycrystalline silicon is initially melted within the crucible and thesilicon ingot is withdrawn from the melt until the melted silicon withinthe crucible is depleted. Alternatively, the ingot may be withdrawn in acontinuous Czochralski method in which polysilicon is intermittently orcontinuously added to the melt to replenish the silicon melt duringingot growth.

In a continuous Czochralski method, the crucible may be divided intoseparate melt zones. For example, the crucible assembly may include anouter melt zone in which polycrystalline silicon is added and melted toreplenish the silicon melt as the silicon ingot grows. The silicon meltflows from the outer melt zone to an intermediate zone within the outermelt zone in which the melt thermally stabilizes. The silicon melt thenflows from the intermediate zone to a growth zone from which the siliconingot is pulled.

Crystal pulling systems may include a heat shield disposed above thecrucible and the silicon melt. The heat shield includes a passagethrough which the silicon ingot passes as it is drawn vertically fromthe silicon melt. The heat shield protects and shields the drawn ingotfrom radiant heat from the melt.

During the melting phase, a temperature gradient may be created withinthe crystal pulling system. The temperature gradient creates thermalstress in the crucible resulting in damage, and in some cases,destruction of the crucible.

A need exists for crystal pulling systems that maintain a more uniformtemperature gradient during meltdown to reduce crucible damage duringmeltdown.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the disclosure, which aredescribed and/or claimed below. This discussion is believed to behelpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

SUMMARY

One aspect of the present disclosure is directed to a crystal pullingsystem for growing a monocrystalline ingot from a silicon melt. Thesystem includes a pull axis and a housing defining a growth chamber. Acrucible assembly is disposed within the growth chamber for containingthe silicon melt. A heat shield defines a central passage through whichan ingot passes during ingot growth. The System includes a cover memberthat is moveable within the heat shield along the pull axis. The covermember includes one or more insulation layers.

Another aspect of the present disclosure is directed to a method forpreparing a melt of silicon in a crucible of a crystal pulling system.The crystal pulling system includes a housing defining a growth chamber,a crucible assembly disposed within the growth chamber for containingthe silicon melt and a heat shield that defines a central passagethrough which an ingot passes during ingot growth. A charge of solidpolycrystalline silicon is added to the crucible assembly. A covermember is lowered through the central passage defined by the heat shieldto cover at least a portion of the charge. The silicon charge is heatedto produce a silicon melt in the crucible assembly while the covermember covers a portion of the charge. The cover member is raised afterthe melt has been formed.

Yet another aspect of the present disclosure is directed to a crystalpulling system for growing a monocrystalline ingot from a silicon melt.The system has a pull axis and includes a housing defining a growthchamber. A crucible assembly is disposed within the growth chamber forcontaining the silicon melt. The system includes a heat shield thatdefines a central passage through which an ingot passes during ingotgrowth. A cover member is moveable within the heat shield along the pullaxis. The cover member includes a first plate having a first plate axisthat is parallel to the pull axis. The cover member includes a secondplate having a second plate axis that is parallel to the pull axis. Thesecond plate is disposed above the first plate.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a crystal pulling system for growing amonocrystalline ingot from a silicon melt;

FIG. 2 is a cross-section view of a portion of the crystal pullingsystem, including a cover member disposed within a central passage of aheat shield;

FIG. 3 is a perspective view of a cover member of the crystal pullingsystem;

FIG. 4 is a cross-section view of the cover member;

FIG. 5 is an assembly view of the cover member;

FIG. 6 is a perspective view of a first plate of the cover member;

FIG. 7 is a bottom view of the first plate;

FIG. 8 is a cross-section view of the first plate;

FIG. 9 is a perspective view of a second plate of the cover member;

FIG. 10 is a top view of the second plate;

FIG. 11 is a cross-section view of the second plate;

FIG. 12 is a perspective view of an insulating layer of the covermember;

FIG. 13 is a cross-section view of a shaft of the cover member;

FIG. 14 is top view of the shaft;

FIG. 15 is a perspective view of a chuck of the crystal pulling system;

FIG. 16 is a perspective view of the chuck engaged with the shaft of thecover member;

FIG. 17 is a graph of the internal temperature of an outer crucible of anested crucible assembly when a cover member with and without insulationis used during meltdown;

FIG. 18 is a graph of the internal temperature of a middle crucible of anested crucible assembly when a cover member with and without insulationis used during meltdown;

FIG. 19 is a graph of the internal temperature of an innermost crucibleof a nested crucible assembly when a cover member with and withoutinsulation is used during meltdown; and

FIG. 20 is a graph of a power profile when a cover member with andwithout insulation is used during meltdown.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

Provisions of the present disclosure relate to a crystal pulling systemfor producing monocrystalline (i.e., single crystal) silicon ingots(e.g., semiconductor or solar-grade material) from a silicon melt by thecontinuous Czochralski (CZ) method. The systems and methods disclosedherein may also be used to grow monocrystalline ingots by a batch orrecharge CZ method. With reference to FIG. 1, the crystal pulling systemis shown schematically and is indicated generally at 10. The crystalpulling system 10 includes a pull axis Y₁₀ and a housing 12 defining agrowth chamber 14. A crucible assembly 16 is disposed within the growthchamber 14. The crucible assembly 16 contains the silicon melt 18 (e.g.,semiconductor or solar-grade material) from which a monocrystallineingot 20 is pulled by a pulling mechanism 22 as discussed further below.

The crystal pulling system 10 includes a heat shield 24 (sometimesreferred to as a “reflector”) that defines a central passage 26 throughwhich the ingot 20 passes during ingot growth. In accordance withembodiments of the present disclosure, prior to the ingot 20 being drawnfrom the melt 18, during an initial melting phase, a cover member 100(FIG. 2) is lowered to at least partially cover the solid charge ofpolycrystalline silicon to reduce heat that radiates through the centralpassage 26 during meltdown. The cover member 100 is moveable within theheat shield 24 along the pull axis Y₁₀.

FIG. 2 shows a portion of the crystal pulling system 10 with the covermember 100 arranged within the central passage 26 during an initialphase in which the charge is melted (i.e., meltdown phase), prior to theingot 20 being drawn. The crucible assembly 16 includes a bottom 30 andan outer sidewall 32 that extends upwards from the bottom 30. Thecrucible assembly 16 includes a central weir 34 and an inner weir 36that extends upward from the bottom 30. The central weir 34 is disposedbetween the outer sidewall 32 and the inner weir 36. The crucibleassembly 16 includes a crucible melt zone 38 disposed between the outersidewall 32 and the central weir 34. The crucible assembly 16 alsocontains an intermediate zone 40 disposed between the central weir 34and the inner weir 36. The crucible assembly 16 also contains a growthzone 42 disposed within the inner weir 36. The crucible assembly 16 maybe made of, for example, quartz or any other suitable material thatenables the crystal pulling system 10 to function as described herein.Further, the crucible assembly 16 may have any suitable size thatenables the crystal pulling system 10 to function as described herein.The crucible assembly 16 may also include three “nested” crucibles whichhave separate bottoms that together make a bottom and in which thesidewalls of the crucibles are the weirs 34, 36 described above.

During ingot growth, polycrystalline silicon is added to the cruciblemelt zone 38 where the silicon melts and replenishes the silicon melt.Silicon melt flows through a central weir opening 44 and into theintermediate zone 40. The silicon melt then flows through an inner weiropening 41 to the growth zone 42 disposed within the inner weir 36. Thevarious silicon melt zones (e.g., melt zone 38, intermediate zone 40 andgrowth zone 42) allow the ingot to be grown in accordance withcontinuous Czochralski methods in which polycrystalline silicon iscontinuously or semi-continuously added to the melt while an ingot 20 iscontinuously pulled from the growth zone 42. The silicon melt 18 withinthe growth zone 42 is contacted with a single seed crystal 75 (FIG. 1).As the seed crystal 75 is slowly raised from the melt 18, atoms from themelt 18 align themselves with and attach to the seed to form the ingot20.

The crucible assembly 16 is supported by a susceptor 50 (FIG. 1). Thesusceptor 50 is supported by a rotatable shaft 51. A side heater 52surrounds the susceptor 50 and crucible assembly 16 for supplyingthermal energy to the system 10. One or more bottom heaters 62 aredisposed below the crucible assembly 16 and susceptor 50. The heaters52, 62 operate to melt an initial charge of solid polycrystallinesilicon feedstock, and maintain the melt 18 in a liquefied state afterthe initial charge is melted. The heaters 52, 62 also act to melt solidpolycrystalline silicon added through feed tube 54 (FIG. 1) duringgrowth of the ingot. The heaters 52, 62 may be any suitable heaters thatenable to system 10 to function as described herein (e.g., resistanceheaters).

The crystal pulling system 10 includes a gas inlet (not shown) forintroducing an inert gas into the growth chamber 14, and one or moreexhaust outlets (not shown) for discharging the inert gas and othergaseous and airborne particles from the growth chamber 14. The gas inletsupplies suitable inert gases such as argon.

The system 10 includes a cylindrical jacket 57 disposed with the heatshield 24. The jacket 57 is fluid-cooled and includes a jacket chamber60 that is aligned with the central passage 26. The ingot 20 is drawnalong the pull axis Y₁₀, through the central passage 26 and into thejacket chamber 60. The jacket 57 cools the drawn ingot 20.

The heat shield 24 is generally frustoconical in shape. The heat shield24 includes an outer surface 61 which faces the crucible assembly 16 andthe melt 18. The heat shield 24 may be coated to prevent contaminationof the melt. In some embodiments, the heat shield 24 is made of twographite shells that include molybdenum sheets therein. The surface 61may be coated (e.g., SiC) to reduce contamination of the melt.

The heat shield 24 includes a bottom 58 (FIG. 2). The central passage 26of the heat shield 24 has a diameter D₂₆ at the bottom 58 of the heatshield 24. The heat shield 24 is disposed above the crucible assembly16, such that the central passage 26 is arranged directly above thegrowth zone 42 so that the ingot drawn from the melt 18 may be pulledthrough the central passage 26. The passage diameter D₂₆ is sized toaccommodate the diameter of the ingot 20 (e.g., 200 mm or 300 mm orother diameter ingots).

The outer surface 61 may be coated with a reflective coating whichreflects radiant heat back towards the melt 18 and the crucible assembly16. As such, the heat shield 24 assists in retaining heat within thecrucible assembly 16 and the melt 18. In addition, the heat shield 24aids in maintaining a generally uniform temperature gradient along thepull axis Y₁₀.

During the initial melting phase, an initial amount of solidpolycrystalline silicon is loaded to a crucible melt zone 38,intermediate zone 40 and growth zone 42. In other embodiments, solidpolycrystalline silicon is added to only one or two of the zonesselected between the crucible melt zone 38, intermediate zone 40 andgrowth zone 42. During meltdown, the cover member 100 is lowered tocover at least a portion of the silicon charge while the initial chargeis melted (i.e., by occluding the central passage 26 of the heat shield24). The pulling mechanism 22 raises and lowers the cover member 100.

In accordance with embodiments of the present disclosure, the covermember 100 is lowered to within less than 30 mm from the bottom 58 ofthe heat shield 24 (i.e., from below or above the bottom 58), or lessthan 20 mm, less than 10 mm, or less than 5 mm from the bottom 58 of theheat shield 24. In some embodiments, the cover member 100 is loweredsuch that it is aligned with the bottom 58 of the heat shield 24. Insome embodiments, the cover member 100 is lowered to within 80 to 100 mmof the surface of the charge during melt down.

After the initial amount of silicon charge has been melted, a secondaryamount of polycrystalline silicon may be added to the crucible melt zone38 (e.g., continuously addled until the entire secondary amount isadded). In accordance with some embodiments of the present disclosure,the cover member 100 covers the central passage 26 while this secondaryamount of polycrystalline silicon is added to the melt zone 38 andmelted down. After the secondary charge has melted, the cover member 100is raised by the pulling mechanism 22. In other embodiments, the covermember 100 is not used while the secondary amount of polycrystallinesilicon is added.

An embodiment of the cover member 100 is shown in FIG. 3. The covermember 100 includes a first plate 102 and a second plate 104 (which mayalso be referred to herein as “lower plate 102” and “upper plate 104”,respectively). Each plate 102, 104 has a central axis that is generallyparallel to the pull axis Y₁₀ (FIG. 1). The first plate 102 and thesecond plate 104 are generally parallel. The second plate 104 isdisposed above the first plate 102.

The first plate 102 includes a first annular wall 106 (FIGS. 5-6) andthe second plate 104 includes a second annular wall. 108 (FIG. 9).Referring now to FIG. 5, the first wall 106 includes a first shoulder110 and a first lip 111. The second wall 108 includes a second shoulder112 and second lip 113. When assembled, the second shoulder 112 rests onthe first lip 111 and the second lip 113 rests on the first shoulder110. A cover member chamber 116 (FIGS. 8 and 11) is disposed between thefirst and second plates 102, 104.

An insulation layer 130 (FIG. 4) is disposed within the chamber 116formed between the first and second plates 102, 104. The insulationlayer 130 has a thickness of T₁₃₀ (e.g., 10 mm to about 50 mm). Theinsulation layer 130 may be compressed between the first plate 102 andthe second plate 104. The insulation layer 130 may include severalstacked layers of insulation or may be a single layer. The insulationlayer 130 may include an opening 132 formed therein.

The insulation layer 130 may be made of felt. The felt may be composedof natural or synthetic fibers. The felt may be purified felt (e.g.,with a max ash of 30 ppm). The insulation layer 130 may generally becomposed of any material that includes suitable insulating properties.

The first plate 102 includes a hub 145 (FIG. 4) that protrudes upwardfor connecting a shaft 150. The second plate includes a second plateopening 128 (FIG. 9). The hub 145 extends through the opening 128 of thesecond plate 104 and through the insulation opening 132 (FIG. 12). Thehub 145 includes a ledge 149 (FIG. 6) with the second plate 104 beingseated on the ledge 149. The hub 145 includes a hub opening 153 (FIG. 8)through which the shaft 150 extends. The hub opening 153 has a profilethat matches the profile of the shaft 150 (e.g., square or rectangularas in the illustrated embodiment or other shape such as circular). Thehub 145 includes a hub chamber 126 having a top wall 157.

The cover member 100 is generally in the shape of a circular segmenthaving a circular portion 120 (FIG. 7) including a center X and acircumference 122 and having a linear edge 124. Specifically, the firstand second plates 102, 104 have the shape of a circular portion with asegment along a chord that has been removed. The first and second plates102, 104 include a major length L₁ and a minor length L₂. The majorlength L₁ is a diameter of the circular portion 120 and the minor lengthL₂ extends from the circumference 122, through the center X to thecircumference 122 of the circular portion 120. The first and secondplates 102, 104 are shaped as a circular segment to allow thecharge/melt to be viewed. In other embodiments, the cover member 100 isfully circular.

In some embodiments, the diameter of the cover member 100 is at least0.75 times the diameter of the central passage 26 at the bottom 58 ofthe heat shield 24 or, as in other embodiments, at least 0.8 times, atleast 0.9 times, at least 0.95 times, or at least 0.99 times thediameter of the central passage 26 at the bottom 58 of the heat shield24.

In some embodiments, the first plate 102 and second plates 104 are madeof graphite. The graphite may be coated with silicon carbide (SiC). Thefirst and second plate 102, 104 may be composed of other suitablematerials. The first and second plates 102, 104 have any suitablethickness T₁₀₂, T₁₀₄ (FIGS. 8 and 11) that prevents thermal stresseswhich result in cracking or damage of the first and second plates 102,104 (e.g., thickness between 3 mm and 50 mm).

With reference to FIGS. 13-14, the cover member 100 includes a shaft 150that supports the cover member 100. The shaft 150 may be connected tothe first and/or second plates 102, 104 in any suitable couplingarrangement. In the illustrated embodiment, the shaft 150 includes anelongated rectangular portion 154 and a collar 156. The collar 156 has adiameter D₁₅₆ less than a diameter of the hub chamber 126 (FIG. 8) andgreater than the width of the hub opening 153. The first plate 102 restson the collar 156. The insulation layer 130 and second plate 104 (FIG.4) are supported by the first plate 102. Alternatively, the shaft 150may be formed integrally with either or both of the first plate 102and/or second plate 104.

With reference to FIGS. 15-16, the pulling mechanism 22 includes a chuck70 that is raised and lowered along the pull axis Y₁₀. The chuck 70 maybe connected to a pull wire or cable 37 that is raised and lowered by adrive motor (i.e., the pull wire or cable and motor are part of thepulling mechanism 22). The cover member 100 is removably connectable tothe chuck 70. For example, the shaft 150 and the chuck 70 may beconnected using a pin lock. The shaft 150 includes a recess 158 (FIG.13). The shaft 150 is inserted into a bore 72 within the chuck 70, suchthat the recess 158 is contained within the bore 72. The chuck 70includes an opening 74 that extends generally perpendicularly to thebore 72, passing through the chuck 70 and opening into the bore 72. Apin 76 is inserted through the opening 74 and into the bore 72 such thatthe pin 76 becomes engaged with the recess 15A of the shaft 150 that isdisposed within the bore 72. In this manner the shaft 150 and the chuck70 are coupled. The shaft 150 and chuck 70 may include any alternativeand/or additional features to couple the cover member 100 to the chuck70.

After meltdown, the cover member 100 is disconnected from the chuck 70and the seed crystal 75 (FIG. 1) is connected to the chuck 70. The seedcrystal 75 may include a similar recess, not shown, such that the seed75 may also be coupled and/or uncoupled to the chuck 70. During theingot growth process, the seed 75 is lowered by the pulling mechanism 22into contact with the melt 18 and then slowly raised from the melt 18.The cover member 100 and/or the seed crystal are selectively coupled anduncoupled to the chuck 70 so that the pull mechanism 22 may be used toraise and lower either the cover member 100 and/or the seed crystal 75.

Compared to conventional crystal pull systems, the crystal pull systemsof embodiments of the present disclosure have several advantages. Use ofa cover member that at least partially covers the charge during meltdownacts to reduce radiant heat loss in the vertical direction which reducesthermal stress in the crucible assembly. In embodiments of the presentdisclosure in which the cover member includes insulation disposedtherein, heat loss through the cover member may be reduced. Inembodiments in which the cover member includes insulation, heater powermay be reduced and the lifetime of the crucible can be furtherincreased.

EXAMPLES

The processes of the present disclosure are further illustrated by thefollowing Examples. These Examples should not be viewed in a limitingsense.

Example 1: Comparison of Crucible Temperature when a Cover Member withand without Insulation is Used During Meltdown

Internal temperatures of the crucible assembly were modeled during theinitial meltdown phase when a cover member similar to that shown in FIG.4 was positioned at the bottom of the heat shield. Another cover membersimilar to that of FIG. 4 was used but the cover member did not includeinsulation (i.e., felt). A nested crucible assembly made of threecrucibles was used. The cover member with insulation resulted in a lowertemperature profile compared to the temperature profile of the crucibleassembly when a cover member without insulation was used when thetemperature was determined at the outer crucible/sidewall (FIG. 17), thecentral weir/middle crucible (FIG. 18), and the inner weir/innermostcrucible (FIG. 19). The maximum decrease in temperature was 20° C. whichoccurred in the inner weir (FIG. 19). This decrease in temperaturereduces damage to the crucible assembly.

Example 2: Comparison of the Power Profiles when a Cover Member with andwithout Insulation is Used During Meltdown

The power supplied to the crucible assembly during the initial meltphase (i.e., the power supplied to the heaters of the crystal pullingsystem) was determined when a cover member similar to that shown in FIG.4 was positioned at the bottom of the heat shield and when another covermember similar to that of FIG. 4 was used but the cover member did notinclude insulation. The power supplied using the cover member withinsulation was less than the power supplied using the cover memberwithout insulation (FIG. 20). The maximum power supplied for the covermember without insulation was 5 kW greater than the maximum powersupplied using the cover member with insulation.

As used herein, the terms “about,” “substantially,” “essentially,” and“approximately” when used in conjunction with ranges of dimensions,concentrations, temperatures or other physical or chemical properties orcharacteristics is meant to cover variations that may exist in the upperand/or lower limits of the ranges of the properties or characteristics,including, for example, variations resulting from rounding, measurementmethodology or other statistical variation.

When introducing elements of the present disclosure or the embodiment(s)thereof, the articles “a,” “an,” “the,” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising,”“including,” “containing,” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements. The use of terms indicating a particular orientation (e.g.,“top,” “bottom,” “side,” etc.) is for convenience of description anddoes not require any particular orientation of the item described.

As various changes could be made in the above constructions and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying drawing[s] shall be interpreted as illustrative and not ina limiting sense.

What is claimed is:
 1. A crystal pulling system for growing amonocrystalline ingot from a silicon melt, the system having a pull axisand comprising: a housing defining a growth chamber; a crucible assemblydisposed within the growth chamber for containing the silicon melt; aheat shield that defines a central passage through which an ingot passesduring ingot growth; and a cover member that is moveable within the heatshield along the pull axis, the cover member comprising one or moreinsulation layers.
 2. The crystal pulling system as set forth in claim 1wherein the cover member comprises a first plate having a first plateaxis that is parallel to the pull axis; and a second plate having asecond plate axis that is parallel to the pull axis, the second platebeing disposed above the first plate, the insulation layer beingdisposed between the first plate and the second plate.
 3. The crystalpulling system as set forth in claim 2 wherein the first and secondplates are both made of graphite.
 4. The crystal pulling system as setforth in claim 3 wherein the first and second plates are silicon carbidecoated.
 5. The crystal pulling system as set forth in claim 1 whereinthe heat shield has a bottom, the central passage of the heat shieldhaving a diameter at the bottom of the heat shield, the cover memberhaving a diameter, wherein the diameter of the cover member is at least0.75 times the diameter of the central passage at the bottom of the heatshield.
 6. The crystal pulling system as set forth in claim 1 whereinthe crystal pulling system comprises a pulling mechanism comprising achuck, the pulling mechanism capable of raising and lowering a chuckalong the pull axis, the cover member being removably connectable to thechuck.
 7. The crystal pulling system as set forth in claim 6 wherein thechuck is connectable to a seed crystal for initiating ingot growth. 8.The crystal pulling system as set forth in claim 1 wherein the crucibleassembly comprises a bottom, an outer sidewall, and an inner weir thatextends upward from the bottom.
 9. The crystal pulling system as setforth in claim 8 wherein the crucible assembly comprises a central weirdisposed between the outer sidewall and inner weir.
 10. The crystalpulling system as set forth in claim 9 wherein the crucible assemblycomprises three nested crucibles.
 11. A method for preparing a melt ofsilicon in a crucible of a crystal pulling system, the crystal pullingsystem comprising a housing defining a growth chamber, a crucibleassembly disposed within the growth chamber for containing the siliconmelt and a heat shield that defines a central passage through which aningot passes during ingot growth, the method comprising: adding a chargeof solid polycrystalline silicon to the crucible assembly; lowering acover member through the central passage defined by the heat shield tocover at least a portion of the charge; heating the silicon charge toproduce a silicon melt in the crucible assembly while the cover membercovers a portion of the charge; and raising the cover member after themelt has been formed.
 12. The method as set forth in claim 11 whereinthe crucible assembly comprises a bottom, an outer sidewall, an innerweir that extends upward from the bottom, and a central weir disposedbetween the outer sidewall and inner weir.
 13. The method as set forthin claim 12 comprising adding polycrystalline silicon to a crucible meltzone disposed between the outer sidewall and the central weir, thesilicon melt flowing through a central weir opening to an intermediatezone disposed between the central weir and the inner weir, the siliconmelt flowing through an inner weir opening to a growth zone disposedwithin the inner weir, the cover member covering at least a portion ofthe silicon melt while adding polycrystalline silicon to the cruciblemelt zone.
 14. The method as set forth in claim 11, wherein the covermember is lowered to within less than 30 mm from a bottom of the heatshield.
 15. The method as set forth in claim 11, wherein the covermember is lowered to a bottom of the heat shield.
 16. The method as setforth in claim 11 where the cover member includes an insulation layerand a silicon carbide coated graphite plate.
 17. A method for forming asingle crystal silicon ingot comprising: preparing a melt of silicon ina crucible of a crystal pulling system by the method of claim 11; andlowering a seed crystal to contact the melt after the cover member hasbeen raised.
 18. The method as set forth in claim 17 wherein thecrucible assembly comprises a bottom, an outer sidewall, an inner weirthat extends upward from the bottom, and a central weir disposed betweenthe outer sidewall and inner weir, the method comprising addingpolycrystalline silicon to a crucible melt zone disposed between theouter sidewall and the central weir, the silicon melt flowing through acentral weir opening to an intermediate zone disposed between thecentral weir and the inner weir, the silicon melt flowing through aninner weir opening to a growth zone disposed within the inner weir, thecover member covering at least a portion of the silicon melt whileadding polycrystalline silicon to the crucible melt zone.
 19. The methodas set forth in claim 17 wherein the cover member and seed crystal areraised and lowered by a pulling mechanism comprising a chuck, the methodcomprising: disconnecting the cover member from the chuck after thecover member is raised; and connecting the seed crystal to the chuckafter the cover member is disconnected from the chuck.
 20. A crystalpulling system for growing a monocrystalline ingot from a silicon melt,the system having a pull axis and comprising: a housing defining agrowth chamber; a crucible assembly disposed within the growth chamberfor containing the silicon melt; a heat shield that defines a centralpassage through which an ingot passes during ingot growth; and a covermember that is moveable within the heat shield along the pull axis, thecover member comprising: a first plate having a first plate axis that isparallel to the pull axis; and a second plate having a second plate axisthat is parallel to the pull axis, the second plate being disposed abovethe first plate.